1. Field of the Invention
The present invention relates to the smart foam for active noise control in a duct. In particular, the present invention relates to the smart foam for active noise control in a duct which may be easily installed in ring type tubes and which can improve the noise control performance within a duct with fluid""s movements.
2. Description of the Related Art
As the noise issue becomes important, the noise problem within ducts used in various machinery and equipment is becoming important increasingly. Until now, the noise in ducts has been controlled by applying noise cut-off materials or absorbing materials, or by attaching devices such as a resonator. While the cut-off material or absorbing material may decrease the noise throughout a broad bandwidth of frequencies, such material may decrease only a limited extent of noise at a low frequency area. On the other hand, the resonator may control only the noise of a certain limited frequencies.
Generally, elastic porous materials reduce noise by dissipating the energy of the incident sound wave, using the friction related to the coupling of the solid constituting the frame and the fluid (air) in the holes. The method of passive noise control using the elastic porous material is the method to absorb sounds merely by installing the elastic porous material between the noise origin and the sound receipt point. This method is simple in structure and easy to implement. Thus, this method has been widely used because of the low cost incurred in installing the device and the broad frequency bandwidth in which noise may be reduced. However, this method has a drawback in that it can hardly control noises of low frequencies.
Accordingly, it was required to develop a new method for noise control and thus the active noise control (ANC) has been studied. Paul Lueg of Germany proposed the basic principle of the active noise control in 1936. Basically, this method reduces noise by generating the control sound, which has the same magnitude as the noise to be controlled and the phase difference of 180 degree from the noise.
For such active noise control, an origin for the control sound is required. Speakers are frequently used for such purpose. However, in order to attach a speaker, the shape of duct must be changed or a portion of the duct must be set aside as a space for the speaker.
In order to resolve such problems, Fuller suggested a smart foam which may generate the control sound. Such smart foam may be created by inserting polyvinylidene fluoride film (PVDF film) inside of the conventional elastic porous foam. The embedded PVDF film serves as an actuator to generate the control sound. The sound is generated by the changes caused upon the fluid and solid of the elastic porous foam. Thus, the smart foam has the advantages of both the passive noise control and the active noise control of the conventional elastic porous material.
FIG. 1 and FIG. 2 illustrate the structure of the smart foam proposed by Fuller.
As illustrated in the drawing, Fuller""s smart foam (xe2x80x9cPlug type smart foamxe2x80x9d) has wavy surfaces (12)(22) of the continuous semicircles on the surfaces of the upper foam (10) and the lower foam (20), facing each other. The upper and lower foams are made out of the elastic porous synthetic resin foam. A PVDF actuator (30), which may radiate sounds, is interposed in between the said wavy surfaces (12)(22) and fixed by the means such as an adhesive.
The said PVDF actuator (30) is made of a wavy PVDF film (32), on both surfaces of which conductive materials (34) such as the silver (Ag) are coated. The said conductive material (34) is peeled off through the lines (36) between two semicircles of the said wavy PVDF film, for the effect of the electric insulator. Thus, the said conductive material (34) is composed of four different and independent cells.
By interposing the semicircular PVDF actuator (30) in the elastic porous material composed of the upper and lower foam parts (10)(20) and applying to the neighboring cells the voltage in the opposite directions, the above-described plug type smart foam enables sounds to be radiated as they are radiated from speakers. Thus, in the low frequency level, the active noise control through the PVDF actuator (30) is possible and in the high frequency level, the passive noise control through the elastic porous material is made passable. However, as shown in the drawing, the plug type smart foam is in the form blocking the cross section of a duct. Thus, it may not be applied to control the noise in a duct with the movement of actual fluid. Moreover, unless both ends are baffled, the sound would radiate as the dipole does. Therefore, the sound radiation efficiency decreases abruptly in the free field.
The purpose of the present invention is to provide the smart foam for active noise control. In particular, the present invention provides the ring type smart form made by inserting a PVDF actuator in the lining shaped ring type elastic porous foam with a hole in the center, which may thus be used in a duct with the movement of fluid inside for the active and passive noise control. Furthermore, the present invention""s smart foam radiates sounds in the manner similar to the monopole does even without baffles. Consequently, the present invention provides the smart foam for active noise control with high sound radiation efficiency even in the free field.
Another purpose of the present invention is to provide a device equipped with the smart foam for active noise control in a duct, in the structure enabling the said ring type smart foam to be easily installed within a ring type duct.
In order to achieve the above-mentioned purposes, the present invention""s smart foam for active noise control in a duct comprises: an external ring of a certain length, made out of the elastic porous synthetic resin foam; an internal ring made out of the elastic porous synthetic resin foam to be inserted inside of the said external ring; a ring type PVDF film, on both sides of which the conductive materials are coated to radiate sounds and which is to be interposed in between the said external and internal rings; and the lead wire which enables the PVDF film to radiate sounds through the connection of the said PVDF film""s inner and outer surfaces to different electrodes and voltages applied to the surfaces.
Furthermore, the device, equipped with the present invention""s smart foam for active noise control in a duct, comprises: a ring type smart foam composed of an external ring of a certain length, made out of the elastic porous synthetic resin foam, an internal ring made out of the elastic porous synthetic resin foam to be inserted inside of the said external ring, a ring type PVDF film, on both sides of which the conductive materials are coated to radiate sounds and which is to be interposed in between the said external and internal rings, and lead wires which enable the PVDF film to radiate sounds through the connection of the conductive materials on the said PVDF film""s inner and outer surfaces to different electrodes and the voltages applied to the conductive material layers; and a holder composed of the first connection part with a hollow for holding the said ring type smart foam and the second connection part extended from the first connection part, having the smaller inner diameter than the said hollow, to which connection parts (the first and second connection parts) the ducts are connected.